1. Introduction

During the last decades the protection systems went through extensive technological advance. Practically the term “relay protection” becomes a bit obsolete, or depicting only the particular sector of automation of the EPS. There is a sustainable tendency more converter based generations to be grid connected [1]. Several cases of severe black-outs [2,3,4,5] imposed increased requirements in respect to EPS protection. The results of post-fault analysis for such events indicated, that normative documents in many countries were in some extent obsolete. In result many new rules and design principles were adopted to enhance the EPS stability, power quality improvement of operation and maintenance flexibility [6,7]. Cyber security is also an important issue for the contemporary EPS automation schemes, and in result of the tendency for transfer to digital technologies, this problem will stand for the future as well [9,10,11]. The electrical/electronic equipment vulnerability to other natural phenomena like extreme solar activity [10] also imposes challenges for the protection systems. “Magnetic storms/solar protuberances” have already impacted the operation of EPS protection and control systems [10]. The protection devices recordings for fault events can be a valuable source of data for the analysis of the pre-history and consequences of the event [8,9,12].

The functionality of the “relay” protection systems can be observed from different aspects. The optimal design solution for particular scheme of protection system can be estimated by different indices as functional applicability, reliability, compatibility with site conditions, maintenance, MMI (Man-Machine Interface) specifics, etc.

In order to achieve an optimal selection of protection devices / IEDs for particular application, object of interest is the characterizing criteria to be systematized and evaluated with “structured” approach. First of all such approach can help to create a “template form” for estimation equipment compliance with user’s requirements. The elaboration of such approach may also ease the selection of equipment out of several alternatives. If preliminary set of requirements / criteria for protection equipment selection (or protection and control) is available, it can be systematized in groups with the respective listed characteristics.

The paper is based on study of 5 different vendors of digital protection devices / IEDs. All studied devices were tested for functional behavior: fault conditions were simulated, connected to PC with firmware for data transfer of settings, logs of events analyzed, etc. The samples of equipment were analyzed in laboratory conditions.

Each of the protection device characteristics can be assigned to a more generalized group, and in such manner they can be presented as:

• Operational principle, functional applicability, applicability for the particular object (scheme);
• Compliance with the respective standards, state /grid operator, utility/ requirements, and client’s specific rules;
• Reliability indices;
• Self-test function, level of self-test, self-test signaling;
• Overall construction – general characteristics;
• Overall construction – internal structure;
• Overall construction – set of functions (electrical, technological protections);
• Intelligent Electronic Devices (IED) characteristics;
• Installation location conditions compatibility;
• Compatibility with power supply;
• Compatibility with peripheral devices;
• Firmware (software);
• Man – machine interface (MMI) characteristics;
• Interfaces to other systems;
• Service time, without necessity of maintenance / servicing;
• Data / event registration; “oscillographic” recorder functions /wave form records/;
• Functionality to operate in centralized/decentralized systems for protection and control;
• Functionality to receive (and redistribute) centralized synchronization;
• Functionality to provide functions for Wide Area Protection (WAP), grid (EPS) level protection functions (load shedding, etc.);
• Functionality for control of commutation apparatus, interlocking between devices, etc.;
• Modular design, options for upgrade and extension;
• Service life, guaranteed under the specified operating conditions;
• Guaranteed period for service, procurement of spare parts, firmware update / support;
• Unauthorized access protection;
• References for application, other;
• Financial aspects.

2. Proposed method for estimation of digital protection devices applicability

In result of study on the vendors proposed devices, the required characteristics and protection devices specifics by users / regulations, generalization was made. The above listed general characteristics (1-26) do not cover all possible categories to estimate the devices specifics. They are presented as general list of IED functionality features, but not pretending to envelope in completeness all applicable characteristics. The qualitative estimation of the level of fulfillment of the designer / investor requirements for each characteristic can be ranked as:

where r_i is the maximal ranking for each characteristic. For the presented example list of 26 groups of generalized characteristics, for each group the value  can be calculated. It can be calculated on basis of the sum of score ranking points  for each sub-characteristic pertaining to the respective group:

where n is the number of grouped “sub-characteristics”.

For equipment evaluation based on 26 generalized characterizing features, can be written:

Where m for the presented set of characteristic groups is 26.

In Table 1 is presented the set of characteristic groups, detailed in sub-characteristics. In the presented proposal for protection equipment estimation 26 groups are included, but definitely this number can be extended or limited in respect to the needed range. In the right column are presented the maximal ranking points, giving options to estimate the 100% criteria fulfillment. For “fulfillment” is assumed the operator’s requirement, accomplishment to achieve the needed protection device functionality.

 Table 1 IED set of characteristic groups

* The reliability indices can be adapted in dependence of the particular application / data available.

The importance of the presented above protection device characteristics depends on the particular case of implementation. If “weighting coefficients” are used as multipliers for each group of parameters, they can have different relative distribution of the ranking values. In some cases not the devices themselves, but the accessories and the software (firmware) can have a decisive importance. The software is a key feature for the client in some

cases. Some companies offer free firmware (for settings, download of records, etc.), some companies offer it as additional option, not free of charge. There are specific occasions, where even a single feature can be a serious obstacle for proper handling of the device: for instance – the device may comply with almost all requirements, but if there is no option for particular language – this could be a critical obstacle for implementation. Some IED producers require payment for the software or part of the software functions. If these expenses are not counted in the “grand total” for protection device / system procurement, the financial comparison of possible options won’t be precise. Depending on the scope of functionality, different software require different computing resources. Some firmware products are supplied to support wide spectrum of devices, but the software complexity can cause slow data transfer, inconvenience during work and extended requirements for PC resources. It is relatively common practice the vendor companies to provide basic operating platform, and the respective software modules for the particular type of protection devices (connectivity packages, product specific libraries) shall be additionally installed.

The software itself can provide different approaches for

handling of the data, visualization screens, etc. The practice shows, that the software which provides informative and clear graphical presentation of the device settings facilitates the work process and reduces the probability for commissioning specialist / operational personnel mistakes.

3. Theoretical results

The scope of criteria presented in Table 1 gives a total maximal “rating” of 2000 points. The distribution of the ranking points between the respective sub-groups shall be revised for a particular case. The specifics of a particular project may impose some features of the selected devices to be more important in conditions of particular application. The content of the set of criteria may also vary.

In order to achieve realistic results by utilization of the proposed approach, it is important to have in mind not only the “quantity” of ranking points in general, but the balance between the accomplishments of the requirements by each position in general. Figuratively, this means that an option for device configuration can have a higher general score than other, but due

Figure 1. Example for comparison of hypothetical options for protection devices selection.

to the facts that some criteria are maximally covered and other are not covered at all. Additional coefficients can be added in order to estimate the distribution of the values between the groups.

In Fig. 1 is presented an example for “application” of the proposed approach for estimation of digital protection devices applicability on basis of multiple characterizing parameters, based on two variants: “Variant 1” and “Variant 2”. The “grand total” score for Variant 1 is 89.42% and for Variant 2 – 92.7%. By absolute values – “Variant 2” shall be preferred. If a “threshold level” of 50% accomplishment is assumed, the screening of the individual groups of characteristics scoring indicates that for selection criteria N 13 – “Variant 2” fails with 40% accomplishment.

The systematized in such manner criteria based on the characteristics of the protection devices (or systems) can be used in two aspects:

• Estimation for equipment compliance to predefined requirements. The presence of particular feature of the protection device and/or compliance of the required parameter(s) brings the respective estimation (analysis of compliance) ranking points. After estimation of all of the features is formed a general (total) sum of ranking points (Es) is calculated. The sum of ranking points can be used as overall estimated criteria for the compliance of the respective device (system) with the preliminary defined criteria.
• Comparison of two (or more) options. The estimation can be made as every characteristic of the different optional types of equipment is compared to reference criteria. Possible approach is also to proceed with mutual comparison of the optional equipment characteristics. This approach can support the selection of optimal variant for protection devices (systems).

Some characteristics can be detailed further in the aspects of functionality to receive / generate / transmit signals for anticipated response. In case of occurrence of faults or other unacceptable processes, such functionality can help reducing the spread of disturbance in the EPS. Channels for accelerated processing of data for anticipated signals from local / remote power generations, priority dispatching centers communication, priority technological signals from the control systems of large concentrated power generations can be used. This option is related to the control systems of the respective objects as well.

Some of the protection devices design related features shall be estimated for a “period in the future”, covering the expected service time. The presence of elements with rotation components like cooling fans, HDDs, can shorten the service life and/or impede the necessity of servicing. Typically the “relay protection” system do not have such elements, but indirectly they can depend on the operation of cabinet ventilation/cooling, power supply units, common SCADA servers, etc. The life span of built-in batteries for real-time clock and other back-up functions can also impose limitations on long-term operation of the electronic devices.

Some of the listed functionalities for protection devices practically are not applicable for electromechanical and solid-state devices. Eventual comparison between electromechanical and microprocessor devices is applicable only for specific cases.

4. Conclusions

The proposed approach for estimation of digital protection devices applicability on basis of multiple characterizing parameters can be used for comparison of alternative options of protection schemes.

The proposed set of criteria for IED applicability estimation, if intended for practical application, shall be “tailored” to the particular grid /grid operator/ utility operator specifics. Some advanced grid protection techniques like application of Phasor Measurement Units (PMU) are still not adopted in many regions, but the swift technology development indicates positive tendency [13]. Such functionality can also be included in the “extended version” of the example given in Table 1. The balanced and perspective designed protection systems will reduce a lot of problems like black-outs, power interruptions, power quality indices, etc. It shall be counted, that applying more sophisticated equipment imposes the necessity of increased installation, maintenance and equipment physical protection practices [14].

On the basis of such approach a procedure can be developed for both technical and economical evaluation of equipment applicability [15]. The proposed frame of criteria (the included in Table 1 characteristics) is based on some of the basic characteristics of the IEDs/protection devices. Extending the scope of criteria may lead to sophisticated ranking approach for detailed review of the characteristics of high technological products. In the scope of criteria the respective relevant IEC/EN/IEEE standards can be included [16,17].

Such approach may support the selection of optimal components for protection system and in the same time to give a sound justification of the selection [18]. If protection devices are selected on the basis of compromise-their limited functionality, insufficient options for upgrade, software limitations, etc., may impose shortages for the operation, maintenance and enhancing the system functionality in the future.

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